1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a plate-like structure, for example, recording media such as a hard disk, a semiconductor, or a magnetic transfer disk.
2. Description of the Related Art
In recent years, significant enhancement of information equipment such as personal computers has sharply increased the amount of information that must be handled by users. In this situation, expectations are being placed on information recording and reproducing apparatuses having a much higher recording density or semiconductor devices having a much higher degree of integration.
A finer machining technique is required to improve the recording density of recording media. Conventional photolithography techniques using an exposure process enable a large area to be micromachined at one time. However, these techniques do not provide a resolution equal to or less than the wavelength of light. Thus, it is difficult to form a microstructure of a size of at most 200 nm using the conventional photolithography technique.
Examples of machining techniques for a size of at most 200 nm include electron beam lithography and focused ion beam lithography. However, disadvantageously, these techniques cannot provide a high throughput.
A “nano imprint lithography (NIL) technique” proposed by S. Y. Chou in 1995 is used to form microstructures of size equal to or smaller than the wavelength of light (see, for example, Appl. Phys. Lett.; Vol. 67 (1995) P3114).
The nano imprint lithography technique involves pressing a master on which fine concave and convex patterns have been formed using the electron beam lithography or the like, against a substrate coated with a resist, to transfer the concave and convex patterns on the master to the resist film.
This technique sharply reduces the time required to process an area of at least 1 square inch compared to the electron beam lithography or focused ion beam lithography.
The steps of nano imprint lithography are as follows.
(1) A resist such as PMMA is applied to a transferred substrate such as a silicon substrate.
(2) The master is pressed against the transferred substrate in a reduced pressure atmosphere. In this case, the pressure is about 100 atm.
(3) The transferred substrate provided with the resist film is heated to at least the glass transition temperature of the resist.
(4) A predetermined time later, the master and the transferred substrate are cooled to room temperature.
(5) The master is stripped off from the transferred substrate.
(6) Concaves and convexes are transferred to the resist film.
Of these steps, the step (3) of heating the transferred substrate to the glass transition temperature of the resist is required to soften the resist to enable the concaves and convexes to be transferred even under a low pressure. However, it takes a long time to heat and cool the transferred substrate, thus lowering throughput. Moreover, the softened resist causes the resist film to attach partly to the master and thus be stripped off from the substrate.
Further, these steps are executed in a reduced pressure atmosphere. This is to prevent transfers from being locally disabled owing to the presence of bubbles between the master and the transferred substrate. However, in forming a reduced pressure atmosphere, a long time is required for deaeration using a pump or the like. This also reduces the throughput.
Further, if the concave and convex patterns on the master are uniformly transferred to a large area of about at least 1 square inch, a high parallelism is required between the surface of the master and the surface of the transferred substrate. Even if the required parallelism is obtained, it is very difficult to uniformly distribute a load over the large area.
As described above, the nano imprint lithography technique is suitable for forming a microstructure of a size equal to or smaller than the wavelength of light. The nano imprint lithography enables a microstructure to be formed with a much higher throughput than a drawing process using the electron beam lithography or focused ion beam lithography.
However, problems with the nano imprint lithography technique are that the throughput is affected by the time required to heat and cool the substrate, that the film may be stripped, that the time required for deaeration affects the throughput, and that it is difficult to ensure the parallelism between the surface of the master and the surface of the transferred substrate and to apply a uniform load to the master and transferred substrate.
In order to solve these problems, the applicant has proposed a room temperature imprint technology (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-157520).
With the room temperature imprint technology, a master and a transferred substrate are sandwiched between a pair of press surfaces under room temperature; the master has a concave and convex region provided with concave and convex patterns and the transferred substrate is coated with a resist. A very high pressure is then applied to the master and transferred substrate to transfer the concave and convex patterns on the master to the resist film on the transferred substrate.
This technique allows the concave and convex formed region of the master except for blank parts to be pressed against the transferred substrate under a uniform pressure. Consequently, fine concave and convex patterns of size 200 nm or less are uniformly transferred to the transferred substrate over a large area. A high throughput is also achieved.
Further, even under atmospheric pressure, when a pressure of at least 500 atm. is applied to the master and transferred substrate, compressed bubbles serve as a protective layer. Accordingly, the master and the transferred substrate are reliably and easily separated from each other to avoid problems such as tear-off of the film.
As described above, the room temperature imprint technique eliminates the disadvantages of the “nano imprint lithography (NIL) technique” proposed by S. Y. Chou. However, the room temperature imprint technique presents the problems described below.
A first problem is that under a pressure of at least 500 atm., elastic deformation of a mold causes a biased abutment phenomenon. This precludes the master and the transferred substrate from being subjected to a uniform pressure.
FIG. 23 is a partly cutaway perspective view of a press machine used for a press process with the room temperature imprint technique.
In FIG. 23, reference numerals 101, 102, and 103 denote a master, a transferred substrate, and an upper mold, respectively. Reference numerals 104 and 105 denote an upper base and a lower mold, respectively. Reference numeral 106 denotes a lower base.
The upper base 104 has a disk-like larger-diameter portion 104a and a disk-like smaller-diameter portion 104b formed on a bottom surface of the larger-diameter portion 104a concentrically with the larger-diameter portion 104a. The upper mold 103 is shaped like a donut and is embedded in a central part of the smaller-diameter portion 104b. 
The lower base 106 is composed of a disk-like larger-diameter portion 106a and a disk-like smaller-diameter portion 106b formed on a top surface of the larger-diameter portion 106a concentrically with the larger-diameter portion 106a. A cylindrical projection 106c is formed so as to extend upward through central holes in the master 101 and transferred substrate 102. The outer diameter of the projection 106c is slightly smaller than the inner diameter of a central hole in the upper mold 10. The lower mold 105 is shaped like a donut and embedded around the periphery of the projection 106c. 
FIG. 24 is a graph showing the distribution of pressure generated when the master and the transferred substrate were pressurized at 1,000 atm. using the press machine used for the press process with the room temperature imprint technique.
FIG. 24 shows that there is a difference of about 20% in pressure. This indicates that the master and the transferred substrate are not subjected to a uniform pressure.
A second problem is the relative misalignment between the master and the transferred substrate.
FIG. 25A is a sectional view showing the master and transferred substrate before the press process with the room temperature imprint technique. FIG. 25B is a sectional view showing the master and transferred substrate during the press process with the room temperature imprint technique.
As previously described, with the room temperature imprint technique, the master and the transferred substrate are subjected to a high pressure of at least 500 atm. Thus, the master and the transferred substrate contract significantly in a vertical direction (in which the pressure is applied) and expand markedly in a horizontal direction (which is perpendicular to the vertical direction).
A Poisson ratio is the ratio of the amount by which the master and transferred substrate contract in the vertical direction to the amount by which the master and transferred substrate expand in the horizontal direction. Every substance has its specific Poisson ratio. The amount by which the master and the transferred substrate expand in the horizontal direction is in proportion to compressive stress and the Poisson ratio and in inverse proportion to the modulus of longitudinal elasticity.
For example, nickel has a modulus of longitudinal elasticity of 1.995×1011 Pa and a Poisson ratio of 0.31. Glass has a modulus of longitudinal elasticity of 7.200×1010 Pa and a Poisson ratio of 0.30.
Accordingly, if the material for the master 101 is nickel and the material for the transferred substrate 102 is glass, the master 101 and the transferred substrate 102 have significantly different moduli of longitudinal elasticity from each other. Consequently, misalignment unavoidably occurs between the master 101 and the transferred substrate 102.
Frictional force acts on the upper mold 103, the lower mold 105, the master 101, and the transferred substrate 102. Thus, with a uniform pressure distribution, the frictional force surpasses a horizontally expanding force to prevent the misalignment between the master 101 and the transferred substrate 102. However, as previously described, the room temperature imprint technique entails a nonuniform pressure distribution. This results in the misalignment between the master 101 and the transferred substrate 102.
FIG. 26 is a graph showing the amount of relative misalignment between the master and the transferred substrate observed if the room temperature imprint technique is used. In FIG. 26, a denotes the amount of misalignment attributed to the room temperature imprint technique, and b denotes a line corresponding to a misalignment amount of zero.
FIG. 26 shows that the amount of misalignment between the master and the transferred substrate is about 20 nm. This misalignment amount is impermissible if patterns of a size of at most 200 nm are to be formed.
It is an object of the present invention to solve problems (1) to (5) listed below and accompanying the nano imprint lithography technique as well as problems (7) to (6) listed below and accompanying the room temperature imprint technique.
(1) A long time is required to heat and cool the substrate, thus lowering the throughput.
(2) When the master is stripped off from the transferred substrate, the resist film may be stripped off from the transferred substrate.
(3) A long time is required deaeration, thus lowering the throughput.
(4) It is difficult to ensure the parallelism between the surface of the master and the surface of the transferred substrate and to apply a uniform pressure to the master and transferred substrate.
(5) During heating or cooling, a difference in coefficient of thermal expansion may result in the relative misalignment between the master and the transferred substrate.
(6) A nonuniform pressure is exerted on the transferred substrate.
(7) A difference in material may lead to the relative misalignment between the master and the transferred substrate.